A Missouri trooper ordered a blood test for a suspected drunk driver who had refused one, without having a warrant. US Supreme Court said Tuesday it will decide if that action was justified. The case could help define the scope of protections against unreasonable searches.

Washington — The US Supreme Court announced on Tuesday that it will take up a case examining whether police officers need a warrant before administering an involuntary blood test to a suspected drunk driver.

Although the case deals with a relatively routine interaction between police and motorists, the underlying legal issue will help define the scope of Fourth Amendment protections against unreasonable searches involving forced blood tests.

The case, Missouri v. McNeely (11-1425), stems from an October 2010 traffic stop by a Missouri state highway patrol officer near Cape Girardeau.

The officer pulled Tyler McNeely over at about 2 a.m. for allegedly exceeding the speed limit by 11 miles per hour. During the routine stop, Officer Mark Winder noticed signs that Mr. McNeely might be intoxicated. He asked the driver to step out of the car and take four field sobriety tests. After McNeely performed poorly, the trooper asked the driver to submit to an alcohol breath test.

The trooper then transported McNeely to a medical clinic. After the driver refused to submit voluntarily to a blood test, Officer Winder directed a clinic staff member to draw blood without the suspect’s permission.

The test showed McNeely’s blood-alcohol level was well above the legal limit. He was charged with driving while intoxicated.

If convicted, it would be McNeely’s third DWI offense and bring a potential four-year prison term.

Prior to his trial, McNeely’s lawyer filed a motion to suppress the blood sample on grounds that the arresting officer had failed to first obtain a search warrant.

Prosecutors fought the motion, arguing that the officer was justified in ordering the blood-test immediately without a warrant because the longer he waited to perform the test the more the alcohol in the suspect’s system would be metabolized. In effect, any delay would mean the continued destruction of evidence.

Officer Winder also stated later that he had read a recent article that Missouri law was changed to waive the requirement to obtain court-authorization prior to performing a blood test.

The trial judge agreed with McNeely’s lawyer and barred prosecutors from using the blood test results.

A state appeals court reversed. But the Missouri Supreme Court ultimately affirmed the trial judge’s decision that the police were not justified in ordering a blood test without first obtaining a warrant.

The Missouri Supreme Court based its decision on a 1966 US Supreme Court precedent that allowed police under certain exigent circumstances to conduct a warrantless blood test.

In the 1966 case, the officer was dealing with the aftermath of a traffic accident. He had to investigate the accident scene and transport the defendant to the hospital for treatment. Under those “special facts,” the US Supreme Court said there was no time for the officer to contact a judge and obtain a search warrant prior to drawing the necessary blood for the blood-alcohol test.

In the McNeely case, there was no accident and no need to transport anyone to the hospital for emergency treatment. Instead, the question in the McNeely case is whether police officers are justified in ordering a warrantless blood test solely because any delay in performing such a test will result in the body’s natural destruction of the evidence.

State supreme courts have reached divergent decisions on that issue. High courts in Wisconsin, Minnesota, and Oregon have ruled that the rapid dissipation of alcohol in the bloodstream creates a sufficient exigency to justify a blood test without a warrant.

In contrast, the Utah Supreme Court and the Iowa Supreme Court have ruled that additional “special facts” are needed beyond just the natural process of dissipation to justify a warrantless blood test.